This invention relates to novel semi-permeable membranes useful for separating one or more gases from one or more other gases.
In various industries, it is necessary or highly desirable to separate one component from another in a gaseous stream. Processes used to perform such separations include pressure swing absorption, cryogenics, and membrane separations. In a membrane separation, a gaseous stream containing the components to be separated is contacted with a membrane, wherein the membrane separates two regions in a manner such that only those materials which permeate through the membrane can communicate from one region to the other. Such membranes are semi-permeable, in that one or more component of the gaseous mixture selectively permeates through the membrane at a rate much higher than one or more of the components in the gaseous stream. Among such separations are the separation of oxygen from nitrogen, and carbon dioxide from methane. The gaseous mixture is contacted with the membrane in a manner such that the selectively permeable species is preferentially transported through the membrane to the other region. The component which is non-selectively permeable may permeate through the membrane but at a much slower rate than the selectively permeable species. It is this difference in rates of permeation which is used to separate the gaseous species or reduce the concentration of the less selectively permeated species in the region to which the permeating gases permeate, or decrease the concentration of the more selectively permeated gas in the region from which the permeating gases permeate.
In such separations, the relative rate of permeation, that is, the difference in rate of permeation between the selectively permeating gas and the non-selectively permeating gas, is a major factor in the separation achieved. The higher the ratio of permeation of the selectively permeable gas over the non-selectively permeable gas, the better the membrane will perform. Therefore, it is desirable to have as high a ratio as possible.
Another important property of membranes is the permeability of the gases through the membrane. If the permeability is too low, the membrane may not provide enough flow through the membrane to be economical for separations. Some potential candidates for membrane separations provide good separation factors but low permeabilities for dense membranes. Flux is the volumetric flow of gas through a particular membrane for a unit area and time, and indicates the productivity of the membrane. The separation factor is the ratio of the permeabilities of the selectively permeating species over the non-selectively permeating species. One technique used to improve the flow of the permeating gases through the membrane is to form asymmetric membranes from such polymers. Asymmetric membranes comprise a membrane with a thin dense region wherein the separation is effected, and a larger region which is porous through which gases pass with little resistance which provides support for the thin dense layer. The discriminating region is much thinner than a solid or homogeneous membrane can be, as the porous layer provides the structural integrity of the membrane and supports the thin dense, layer. This thin, dense layer is located on one surface of the membrane. The formation of an asymmetric membrane with good separation factors and permeabilities is a difficult chemistry and engineering problem. As the thin, dense layer is on one of the surfaces of the membrane, this thin, dense layer is subject to being damaged by handling or exposure to contaminants. This damage can result in leaks in the membrane and render the membrane less effective in separating gases.
Presently, membranes derived from acetate esters, for example cellulose diacetate, and cellulose triacetate, polyamides, polyimides, and olefins, for example polyethylene, polypropylene, poly-4-methylpentene-1, are used for gas separations. Recently it has been discovered that bisphenol based polycarbonates, and polyestercarbonates wherein at least 25 percent by weight of the bisphenol moieties are tetrahalogenated, wherein the halogen is Cl or Br, exhibit excellent separation factors for the separation of oxygen from nitrogen, but exhibit low flux in the dense form.
What are needed are membranes with regions capable of separating one or more gases from one or more other gases which have both good separation factors and flux. What are further needed are membranes which have such regions which are not subject to damage due to handling or exposure to contaminants. What are further needed are membranes which exhibit good physical properties.